1. Field of the Invention
The present invention relates to write-once type optical disks.
2. Description of the Related Art
An optical disc is one type of information recording media. Most conventional write-once optical disks such as CD-R and DVD-R have the following structure. The face of a track side (groove side) provided on a transparent resin substrate is doped with an organic dye by spin coating to form an organic dye layer over the track. A metallic reflection layer made from, for example, gold, silver, silver alloy, aluminum, or aluminum alloy is formed on the organic dye layer. Then, a protective coat (referred to as a “cover layer” hereinafter) made from, for example, an ultraviolet light cure resin is formed on the metallic reflection layer. When recording data (information) on the optical disk, a laser beam is radiated on the substrate side. The later beam decomposes the organic dye. The refractive index is changed by the decomposition of the organic dye. The laser beam also deforms the track and/or the reflection layer due to heat generation. The refractive index change, the track deformation and the reflection layer deformation in combination create so-called pits to record the information.
In recent years, a next-generation recordable optical disk system (referred to as “DVR” system) has been developed, which utilizes a violet laser as a light source for recording and reproducing information together with an objective lens having a high numerical aperture (0.85 or more). In the DVR (Digital Video Recorder) system, the cover layer of the optical disk is about 0.1 mm thick, and the laser beam is radiated on the cover layer side. The DVR system suppresses influences of optical aberration produced by use of the high numerical aperture objective lens, and increases recording density of information, thereby achieving a so-called high capacity information recording disk.
Accordingly, a write once optical disk utilized in the DVR system (referred to as “DVR-R disk” hereinafter) is different from the conventional write once optical disks such as CD-R and DVD-R disks in that the laser beam is radiated to the cover layer side. In other words, the layers are stacked on the substrate in the DVR-R disk in an inverse order, as compared with the layers in the CD-R and DVD-R disk. A method of manufacturing the DVR-R disk is also different from a method of manufacturing the CD-R and DVD-R disks in that when the cover layer is formed, a dielectric layer should be formed in order to protect the organic dye layer from a non-cured ultraviolet light curing resin. The ultraviolet light curing resin is a material which forms the cover layer.
Therefore, the production of DVR-R disks requires a choice of optimized materials and structure.